Ultra-High-Performance Concrete (UHPC) and Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) represent promising materials in the field of construction, offering exceptional strength and durability, making them ideal for the development of a wide range of infrastructure projects. One of the goals is to better understand the impact of each component of the materials on their key properties in the hardened state. This work examines the effect of the aggregate on the properties of UHPC and UHPFRC. This article provides test results for five compositions without fiber, and five compositions with 2% corrugated steel fiber. Three aggregate concentrations (0, 0.2, and 0.4 ${\mathrm{m}}^3$) and quartz sand with different maximum particle sizes (0.4 and 0.8 mm) were selected. It was found that the mechanical properties of the material, such as the steel fiber bond strength, compressive and axial tensile strength, fracture energy, and critical stress intensity factor, depend on both the concentration of the aggregate and the size of its particles. A novel mix-design parameter was proposed, which reflects the total surface area of the aggregate in the composition ($S_{agg,tot}$). The relationships between the parameter $S_{agg,tot}$ and the mechanical characteristics of UHPC and UHPFRC were established. The steel fiber bond strength, axial tensile strength, and fracture energy-related parameters grew non-linearly when the parameter $S_{agg,tot}$ increased. When the parameter $S_{agg,tot}$ was changed from 0 to 12.38 · ${10}^3{ \mathrm{m}}^2$, the fiber bond strength increased by 1.38 times. The axial tensile strength and total fracture energy of the UHPFRC increased by 1.48 and 1.63 times, respectively. The compressive strength changed linearly and increased by 1.12 times. The improvement in the mechanical properties of the material was associated with an increase in the friction force between the fiber and the matrix, which was confirmed by the formation of a greater number of scratches on the surface of the fiber with an increasing value of the parameter $S_{agg,tot}$. The deformation characteristics, such as modulus of elasticity, Poisson’s ratio, and drying shrinkage strain, were determined solely by the volumetric concentration of the aggregate, as in conventional concrete. An increase in the aggregate volume content from 0 to 0.4 ${\mathrm{m}}^3$ led to an increase in the modulus of elasticity of 1.41–1.44 times, and a decrease in the ultimate shrinkage strain of almost 2 times. The dependencies obtained in this work can be used to predict the properties of UHPC and UHPFRC, taking into account the type and volume concentration of the aggregate.

This article describes the influence of corrugated steel fiber on the mechanical properties and fracture energy of Ultra-High Performance Concretes (UHPC) of various strength levels. Three UHPC formulations with compressive strengths of 143, 152, and 177 MPa were tested. The following parameters for the formulations without fiber and those containing 2% steel fiber by volume were determined: compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, Poisson’s ratio and critical stress intensity factor. From the axial tensile test results, the following parameters were obtained: the cracking stress, tensile strength, and fracture energy of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) of different strength levels. With the introduction of steel fiber, an increase in all the investigated parameters is observed regardless of the strength of the concrete matrix. The most remarkable influence the fiber has on the splitting tensile strength, flexural strength and critical stress intensity coefficient, the increase is up to 1.6–3.2 times. There was a slight increase in compressive strength and elastic modulus—up to 5.0–7.4% depending on the composition. Poisson’s ratio was equal to 0.2 regardless of the strength of the concrete matrix and the presence of steel fiber. Based on the test results, equations were proposed to predict the properties of UHPC and UHPFRC depending on the water–cement ratio, silica fume content, cement compressive strength and the volumetric content of corrugated steel fiber. The calculated and experimental values showed good convergence with a correlation coefficient in the range of 0.885–0.997.

This study examines the impact of sodium citrate and a plasticizing additive, along with their sequential introduction into a cement slurry or concrete mix, on the heat evolution of the cement slurry, the microstructure, phase composition of the cement paste, and the compressive strength of fine-grained concrete. The binder used in this research was a blended binder consisting of 90% Portland cement and 10% calcium aluminate cement. This type of binder is characterized by an increased heat evolution and accelerated setting time. The addition of sodium citrate at 5% of the binder mass alters the phase composition of newly formed compounds by increasing the quantity of AFt and AFm phases. The presence of sodium citrate significantly delays the hydration process of tricalcium silicate by a factor of 3.3. Initially, it accelerates belite hydration by 31.6%, but subsequently slows it down, with a retardation of 43.4% observed at 28 days. During the hardening process, the hydration of tricalcium aluminate and tetracalcium aluminoferrite is accelerated throughout the hardening process, with the maximum acceleration occurring within the first 24 h. During the first 24 h of hydration, the dissolution rates of tricalcium aluminate and tetracalcium aluminoferrite were 40.7% and 75% faster, respectively. Sodium citrate enhances heat evolution during the initial 24 h by up to 4.3 times and reduces the induction period by up to 5 times. Furthermore, sodium citrate promotes early strength development during the initial curing period, enhancing compressive strength by up to 6.4 times compared to the reference composition.

The article examines the influence of steel fiber type and volumetric content on the mechanical properties of UHPFRC. Five fiber types were used: corrugated 15/0.3 and 22/0.3 mm, straight 13/0.3 and 13/0.2 mm, and hooked-end 30/0.5 mm. The fiber volume content ranged from 0 to 3%. During the experimental studies, compressive strength, flexural strength, as well as the flexural fracture energy were determined. It has been discovered that an increase in the volumetric content of steel fiber results in an enhancement of all the properties under consideration, irrespective of the type of fiber. The fiber has the biggest effect on the bending fracture energy and the least on the compressive strength. Linear relationships between the mechanical characteristics of UHPFRC and the fiber factor were established, which reflects the volumetric content, length, and diameter of an individual fiber. The slope angle of the approximating lines of the “compressive strength-fiber factor” and “flexural strength-fiber factor” relationships takes on different values depending on the type of fiber used. The highest slope angle is found for corrugated fiber. It was discovered that there is a threshold value of the fiber factor, upon reaching which the steel fiber leads to an increase in the flexural strength of the UHPFRC. According to a generalized quality criterion it has been determined that corrugated fiber with dimensions of 22/0.3 mm is the optimal substitute for straight fiber 13/0.2 mm, which is widely utilized for the production of Ultra-High Performance Fiber-Reinforced Concrete

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) exhibits exceptional axial tensile strength and a plastic fracture behavior distinct from conventional fiber-reinforced concrete. This distinction arises from a strain-hardening phase, characterized by the formation of multiple, uniformly distributed microcracks and an increase in tensile stress beyond the cracking threshold. This study investigates the axial tensile performance of UHPFRC specimens reinforced with varying types and volumetric contents of fibers. Brass-coated corrugated steel fiber with length-to-diameter ratios of 15/0.3 mm and 22/0.3 mm, along with straight fiber with a ratio of 13/0.2 mm, were employed as dispersed reinforcements, with fiber content ranging from 1% to 3% by volume. The findings reveal that neither fiber type nor content significantly influences the cracking stress. However, the maximum tensile stress and fracture energy demonstrate a linear increase with the fiber factor, which integrates fiber volume fraction and geometric characteristics. For equivalent fiber factor values, both corrugated and straight fibers exhibit similar tensile stress, but corrugated fibers contribute to higher fracture energy. Based on the experimental results, an equation was derived to determine the minimum required fiber volume, given specific geometric properties, to achieve strain-hardening behavior under axial tension.

The uniform distribution of steel fibers in self-compacting Ultra-High Performance Steel Fiber Reinforced Concrete is one of the factors that determine the properties of the hardened material. In self-compacting concrete mixes characterized by low yield stress and plastic viscosity, steel fibers can settle under gravity, which can lead to a decrease in strength. In this study, a computational-experimental method is proposed for determining the height of steel fiber sedimentation, as well as the segregation index of a steel fiber reinforced concrete mixture. Mixtures were tested with a superplasticizer content of 1, 1.3, and 1.6% with a slump flow diameter of 288, 356, and 378 mm. According to the results of calculations performed according to the proposed method, it was found that the height of steel fiber sedimentation is 3.1, 5.9, and 22.8 mm in mixtures with a plasticizer content of 1, 1.3 and 1.6%. The calculated value of the segregation index of the steel fiber reinforced concrete is quite close to the experimental one with a correlation coefficient of 0.998. Based on the results obtained, a criterion was proposed for determining the maximum allowable height of fiber sedimentation, taking into account the actual height of the concreted structure, which can be used in the concrete mix design.